Nit: Nits are lice eggs. They are hard to see and are often confused with dandruff or hair spray droplets. Nits are found firmly attached to the hair shaft. They are oval and usually yellow to white. Nits take about a week to hatch. All nits must be removed to prevent reinfestation with lice. They can be removed with a special comb or with the fingers. Topical preparations are available that loosen the "glue" that binds them to the hair, making removal easier.
Incidentally, the term "nit-picking" -- meaning "too much minor, overly particular criticism" -- is relatively recent; it dates only to the 1960s. Nit-picking is in the realm of hairsplitting.
See also pediculosis.